Perimetry is a very important and common test in clinical eye care, second only to the simple determination of visual acuity. It is used for examining the range and the sensitivity of a subject's visual field, e.g. in connection with diagnosis and treatment of glaucoma, for testing for neurological diseases, in mass screening etc.
In static threshold perimetry, the limit or threshold of a subject's light perception at a number of discrete locations in the visual field is determined. The test is conducted by means of a computerized perimeter, typically in the following way: the patient is seated in front of a perimeter and asked to look steadily at a centrally placed fixation target, e.g. on a screen or in a hemispherical bowl. Visual stimuli are presented successively with different intensities and at different locations. The patient is asked to press a response button every time he perceives a stimulus, whether close to or distant from the fixation target, whether faint or strong.
There are different strategies for selecting test locations and intensities of the stimuli presented at these test locations in order to establish a patient's threshold for perception of light. In one common method, a stimulus is shown having an intensity close to the expected threshold value at each test location concerned. If the patient does not respond to the stimulus, the intensity of the subsequently presented stimuli is thereafter increased stepwise until a response is received from the patient, i.e. until a stimulus is seen. The first intensity level at which a response is received can be defined as the threshold of the test location concerned. The precision of the test can be increased by reversing the test process when the first response is received, and by continuing it in smaller steps with decreasing intensities until the first unseen stimulus is encountered. The threshold can then be defined as the average intensity level of the last seen stimulus and the first unseen stimulus. If, on the other hand, the patient responds to the first stimulus, the intensity is decreased stepwise until no response is received, whereupon the test procedure is reversed.
The test locations are usually tested in random order, a stimulus at a first test location being followed by a stimulus at another test location etc., and the next stimulus for the first test locations being not presented until after several subsequent stimulus presentations.
The above method of presenting series of visual stimuli with alternately increasing and decreasing intensities is called the staircase method.
In the staircase method, the intensity steps between stimuli of increasing/decreasing intensities are constant, at least between reversals. A variant hereof is the Robbins-Monroe method where the steps between successive stimuli are gradually decreased.
Another method for determining threshold values is the Modified Binary Search (MOBS), according to which a stimulus with a selected intensity close to the expected threshold value of the subject is presented. If the stimulus is seen, its intensity value is regarded as the upper threshold boundary and, if it is not seen, as the lower threshold boundary. The intensity range is then divided into a series of increasingly smaller half-intervals until the upper and lower treshold boundaries are within a defined range.
When the threshold values for all the test locations in the visual field have been determined, they are often compared with normal, previously determined threshold values for patients of the same age to establish whether there are any deviations from normal, or with previous values for the same eye of the patient to establish whether a disease under observation has progressed or receded.
One problem inherent in static threshold perimetry is that this technique is time-consuming. As described above, several stimuli of different intensities are presented at each test location on the screen. Since 50-100 test points are usually examined, one test consists of several hundred stimulus presentations. Typically, the time required for a complete static threshold perimetry test is about 10-20 minutes per eye. The test is very tiring, which may affect the test accuracy.
Furthermore, even though the test procedure is computerized, an operator must usually be present during the test to ensure that the patient maintains fixation, to encourage him and to answer any questions he may have.
Another problem of static threshold perimetry is the assessment of the responses from the patients. After each presentation of a stimulus, the perimeter waits for a response during at most a predetermined time period. If no response is received during the predetermined time period, the stimulus has usually not been seen, and if a response is received, the stimulus has usually been seen. However, it is well-known that patients sometimes press the button without having seen any stimulus at all, and they sometimes fail to press it despite having seen the stimulus. These kinds of responses are called false positive responses and false negative responses, respectively. It goes without saying that they affect the accuracy of the threshold determination and, hence, have an essential impact on the choice of test strategy.
The frequency of false positive responses may be examined by false positive catch trials. The perimeter acts as when displaying a stimulus but without showing one, and registers whether the patient responds or not. The frequency of false negative responses may be examined by false negative catch trials. A strong, supraliminal stimulus is presented at a point where the threshold has already been measured, and the perimeter registers whether the patient responds or not. Often 20-30 catch trials are presented during a test.
Many suggestions have been made for shortening the total time required for performing static threshold perimetry by reducing the number of presentations of stimuli.
Heijl A, Krakau CET: An automatic perimeter for glaucoma visual field screening and control; Albrecht v. Graefes Arch. Clin exp. Ophthalmol 197:13-22, 1975, and U.S. Pat. No. 4,334,738 describe methods for better selecting the intensity level for the first stimulus presented at each test point, whereby the number of presentations of stimuli required for arriving at a threshold value can be reduced.
U.S. Pat. No. 4,927,259 discloses a test strategy which is based on the above-mentioned staircase method, but where the steps of the staircase are logarithmic instead of constant.
U.S. Pat. No. 5,050,983 discloses a test strategy, where some test points are fully tested by the staircase or MOBS method and the remaining test points are tested by just a few stimulus presentations. First, the threshold values of a few seed points are determined by the staircase method or the modified binary search method. Then, the expected threshold values of the neighbor non-seed points are determined. These expected threshold values are tested in a first phase by presenting a visual stimulus, the intensity of which is either decidedly below or decidedly above the expected threshold value in each of the neighbor points, and by checking that the response is in accordance with the expected response ("not seen" and "seen", respectively). Then, the agreement between the expected response and the actual response for each test point is compared with that of its neighbors, and on the basis of this comparison, the neighbor agreement between each point in the field and its nearest neighbors is classified into a "discrepancy", "low-confidence" or "high-confidence" category. The expected threshold value of test points in the low-confidence category is then adjusted for better neighbor agreement.
In a second phase of the method, a further visual stimulus is presented at each non-seed point. The test points which in the first phase were tested with a visual stimulus above the expected threshold value are now tested with a visual stimulus below the expected threshold value, and vice versa. For test points in the "high-confidence" category, the interval between the stimulus intensity and the expected threshold is decreased. Then, the agreement between the responses and the expected responses is determined and compared with that of its neighbors in the same manner as in the first phase. The testing of test points which after the second phase are in the "high-confidence" category is terminated. The expected threshold values of test points in the "low confidence" category are adjusted for better neighbor agreement, whereupon these test points are tested anew in accordance with the first phase. Test points in the "discrepancy" category are retested by the staircase method or the MOBS method.
Moreover, a doctoral dissertation "Statistics in perimetry", Department of Mathematical Statistics, Lund, 1991, by Jonny Olsson, describes the use of posterior probability functions to estimate threshold values after a conventional static threshold perimetry test has been carried out.